Repetitions

AUG 15, 2012:  With enough repetitions---going slowly enough to get everything---the pianist can learn just about anything within that pianist's skill level, and maybe even a little beyond the current skill level.  If it's way over the skill level of the performer, either adjustments might need to be made (taking the whole thing slower, leaving out notes, making a simplified version) or the performer could wait until further mastery of the instrument has been achieved and then try re-learning the music.

If you have to practice hours just to keep it correct at a certain tempo, then the piece may be beyond your present skill level, but there's no harm in stretching your ability to perform difficult music, provided you take it slowly enough to get all the notes and all the music written on the page.

After a certain amount of repetiton, the passage should feel natural enough so that you can perform it without strain or concern.  But that doesn't mean you don't need to concentrate.  One test you can use as to how well you know the work is to get away from the music and the keyboard and try to imagine the way the piece feels on a keyboard and the way it sounds.  If there are holes in that knowledge, you can then go back to the passage or the bar where your memory is fuzzy by consulting the score and then playing it over.  This may be the place that needs real work.

At any rate, once you get everything worked out, it should sort of click into place.  At the very least, you need to know what's coming up in the next ten seconds or so at any point in time so that you're prepared to perform it.